In the field of communications, it is desirable to send a message from a transmission point, through a channel, and to receive such message at one or more destination points. The message may consist of certain information in the form of discrete numbers, continuous analog waveforms, or the like. Consequently, the message may be as diverse as a digital data transmission between computers or a television broadcast. The channel includes the medium through which the message is sent. A typical channel is the atmosphere, such as in a radio broadcast communication scenario, for example. It has become general practice in the communication field to modulate the information to be sent into a continuous waveform which will be transmitted over the channel. The channel, due to natural phenomena, will alter the continuous waveform in some respects such as adding noise and/or fading in certain frequency ranges and time intervals. In addition, in hostile environments, the waveform may be intercepted or subjected to man-made interference in certain frequency ranges and/or time intervals. As such, recovery of the information transmitted becomes problematic. Moreover, many current communication schemes employ compression techniques for compressing the information to be transmitted, in an effort to save time or reduce bandwidth requirements, which, in turn, increases the possibility of error in recovery.
As illustrated in FIG. 1, the classical configuration for such a communication system involves a modulator 10 at the transmitter, for embedding the data sequence q[n] into a signal x(t) which is transmitted over the channel 12. The data sequence q[n] is the information to be communicated. The channel 12 alters the transmitted signal x(t), producing the distorted signal r(t). At the receiver, a demodulator 14 processes the distorted signal r(t)to extract an optimal estimate q[n] of the data sequence q[n].
Typically, the channel 12 would be open for some time interval T during which it has a particular bandwidth W and signal-to-noise ratio (hereinafter "SNR"). This rather generic model of the channel 12, as illustrated in FIG. 2, can be used to describe both characteristics of the transmission medium, and constraints inherent in one or more receivers, for a multiple receiver application. As illustrated in FIG. 2, z(t) represents the noise which is added by the channel to the transmitted signal x(t). In short, the transmitted signal x(t) is affected by channel bandwidth constraints (indicated by the filter B(w)) and duration constraints (indicated by switch 15), noise characteristics, and receiver processing limitations (which may typically be lumped into the function B(w) and switch 15) in certain applications.
When either the bandwidth or duration parameters or properties of a channel are known before transmission, many well-established approaches for transmitting and receiving information (q[n]) reliably, exist. In a wide range of applications, however, both the bandwidth and duration parameters are either unknown or at least inaccessible to the transmitter. As such, with present communication systems, in such applications, communication may become unreliable at times and accurate communication unattainable.
Such applications, in which both the bandwidth and duration parameters are either unknown or at least inaccessible to the transmitter, encompass both point-to point and broadcast communication scenarios, and include the following:
1. Broadcast communication involving multiple receivers of unknown or differing processing capabilities;
2. Communication involving channels subject to hostile jamming and other interception attempts;
3. Communication involving multiple access channels;
4. Communication involving fading channels; and
5. Covert communication involving desired low probability of intercept.
In a broadcast communication application, involving multiple receivers, the various receivers may have different and diverse processing capabilities which may be described in terms of different bandwidth and/or duration constraints. Additionally, there may exist uncertainty in the bandwidth and duration characteristics of the channel.
In a communication system utilizing a channel subject to hostile jamming, attempted interception and/or transmission blocking occurs within certain unknown frequency ranges and time intervals. Thus, similarly, unknown bandwidth and time-varying constraints are encountered.
In a communication system involving multiple access channels, such as that used in telephone and satellite communication systems, for examples, multiple communication users share each channel. In such applications, there exist only a few known fairly burdensome ways to differentiate among the multiple users. These few known ways include sharing in the time domain, sharing in the frequency domain, coding among users and combinations thereof. As such, the effective time and bandwidth channel restraints to a potential user are similar. Additionally, in communication systems involving packet switching schemes, such as those used in telephone and computer communication systems, packets of information are sent asynchronously. As such, routing problems exist such as potential collisions of such packets. These scenarios can present similar time and bandwidth constraints.
In a communication system involving fading channels, such as meteor burst channels, ocean acoustic channels, or mobile radio channels, strong attenuation exists in certain time intervals and/or frequency ranges. In the meteor burst application, an optimum situation arises when a meteor burst occurs, for transmission of information. This optimum situation, however, is unpredictable and unreliable. In both the ocean acoustic and mobile radio channel scenarios, transmitted signals are subject to reflection and possible refraction and, therefore, both constructive and destructive interference occurs. This constructive and destructive interference is, for the most part, uncontrollable and unreliable. Therefore, these fading channel scenarios can present a situation analogous to that of a channel with unknown bandwidth and duration parameters.
Accordingly, a general purpose of the present invention is to provide a communication system capable of accurate transmission and reception of information.